Fiber-reinforced composite materials make the best use of the features of excellent specific strength and specific elasticity and are widely applied to applications such as aviation/space articles. Conventionally, epoxy resins are primarily used as a matrix resin; however, the epoxy resin suffers from the problem of not being able to sufficiently satisfy the requirement of thermal resistance against 200° C. or higher. On the other hand, although polyimides known as a high-temperature resin are excellent in thermal resistance, they cause a problem in formability and thus their practical use to a matrix resin is behind.
In these situations, bismaleimide resins such as aromatic bismaleimide-based resins excellent in balance between thermal resistance and moldability are paid to attention as a matrix resin for fiber-reinforced composite materials such as carbon fibers. However, bismaleimide resins have the defect of low toughness, and therefore their applications are considerably limited. Although a method of blending a rubber component or a thermoplastic resin and a method of copolymerizing other monomers are proposed as a method of improving this defect of bismaleimide resins, the resin has created problems such as the improvement of toughness being insufficient as compared to a decrease in physical properties such as thermal resistance being large.
For example, Japanese Patent Laid-Open No. 3-197559 (Patent Document 1) discloses a prepreg excellent in impact resistance properties including an aromatic bismaleimide-based resin, alkenylphenol or an alkenylphenoxy group-containing comonomer and a soluble thermoplastic polyimide resin. However, it has been ascertained that thermal oxidation resistance properties and solvent resistance (MEK) characteristics of a fiber-reinforced composite material are insufficient in the disclosed resin composition.
In addition, Japanese Patent Laid-Open No. 8-127663 (Patent Document 2) discloses a prepreg excellent in impact resistance properties including a polyfunctional maleimide-based resin 50% by weight of which is present as a solid, alkenylphenol and/or an alkenylphenol ether compound and a thermoplastic resin molded material, wherein the thermoplastic resin molded material does not dissolve in the resin composition and is present on the prepreg surface. However, it is apparent that the glass transition temperature and thermal oxidation resistance properties of a fiber-reinforced composite material are insufficient in the disclosed resin composition.    Patent Document 1: Japanese Patent Laid-Open No. 3-197559    Patent Document 2: Japanese Patent Laid-Open No. 8-127663